Forced Air Gas Furnaces

Forces air gas furnace is a central heating system used to heat a home or building. All furnaces work on the same basic principles and have similar parts:

  • Thermostat
  • Pilot light or electric igniter
  • Gas valve
  • Burners
  • Heat exchanger
  • Fan
  • Exhaust

The pilot light ignites a series of burners inside the combustion chamber that then enters the heat exchanger, where the heat transfers to the air in the home with the use of a fan to heat the home to the set point of the thermostat.


There are 3 types of forced gas furnaces I come across inspecting homes:

  • Low efficiency furnace 78% or less
  • Mid efficiency furnace 80% – 89%
  • High efficiency furnace 90%-98% (Condensing)

When looking at the efficiency it is easy to understand if you think in these terms:

  • In an 80% efficient furnace 20% of the heat is loss in the combustion and goes out the chimney.

Low and Mid efficient furnaces have one heat exchanger usually 1 gas valve setting, and standard fan adjusted to one speed for heat, cool or just fan. They require a chimney to vent the flue gases to the exterior of the home, with temperatures of up to 300-350 degrees.


High efficiency condensing furnaces will have two heat exchangers, the standard heat exchanger as well as a second heat exchanger the flue gases are run though to extract as much heat from the system as possible. This process does cause condensation that is why they are called condensing furnaces. Because of the secondary heat exchanger these furnaces use PVC exhaust pipes with a temperature of about 100 degrees.


High efficient furnaces also come with several options to choose from to make the furnace even more efficient:

2-Statge gas valve

  • 2 – stage furnace will operate in its low-fire stage during periods of mild weather (pushing less air through the ducts and less combustion gas through the flue), and automatically ramp up to its second high-fire stage during extremely colder weather.
  • This will cause the home to experience fewer and much less severe temperature swings in the home, which will result in greater over-all comfort for you and energy savings.

Variable Speed Motor (ECM) – This is the blower motor in the furnace, unlike the traditional motor is saves you money and adds to the comfort of your home:

  • The motor converts AC current into DC current which is cheaper to run
  • When starting it will ramp up to speed and ramp down to prevent a large use of power at the start
  • When sit to fan mode it will operate the fan at 1/8th the cost of the traditional fan.
  • With the fan running the air is constantly circulating preventing cool or hot areas and giving the home a consistent temperature throughout the home

With a high efficient furnace, I recommend to not turn the thermostat below 60 degrees because of the following possible results:

  • With return air that cold, flue gas will condense in the primary heat exchanger causing rust & corrosion.
  • Newer, more efficient heat exchangers are, by necessity, thinner than they used to be, and if you set the thermostat too low in heating mode, the increased temperature differential between the returned air and the heated air increases the effect of the cool/hot contraction/expansion cycle of the metal in the heat exchanger, which significantly shortens its life

In the event that the furnace is not operating here are a couple recommendations to check before calling an HVAC specialist.

  • Turn off the main switch to reset the control board, remove the air filter and turn back on. If the furnace starts most likely a dirty air filter, install a clean filter. If this does not work check the exterior of the vent pipe for a blockage, if blockage is found turn off main switch then turn back on to reset control board and see if it runs normally.
  • If neither of these works. Call a licensed and insured HVAC contractor for further evaluation and repair or replace as warranted.

Reverse Polarity in Home Wiring

Reverse Polarity is a common problem I come across either in the outlets or with the home’s wiring. Home wiring is color coded, Black is HOT (often called the live wire) and WHITE is NEUTRAL. This is different from 12 volt systems where the black is often the neutral, it is very important to understand how wiring differs for a home over an automobile.

In home wiring it is very easy to know what the wires are:

  • White wire is the neutral
  • Green or bare wire is the ground
  • Any other color than white, green or bare is a hot (live wire)

C:Documents and SettingsGilbertMy Documentsinfo4b.pdf

An outlet that is wired backwards is a reversed outlet, the terminals on a outlet is colored to let you know what is the hot side and what is the neutral. Brass is hot (black wire) the silver is neutral (white wire). If an outlet has reverse polarity is it hard to tell unless it is tested because alternating current (AC) will only travel in one direction like water flowing in the river. If it was hooked up to direct current (DC/12 volt battery) the motor would rotate in the opposite direction and is very easy to recognize.

The reason that reversed polarity can create a shock hazard in certain situations is because the switch is positioned before the hot wire side enters the appliance and the neutral is connected to the other end of the appliance circuitry, when the polarity is reversed the appliance circuitry is electrically charged all the time, but only functional when a switch closes the neutral wire connection and the current begins flowing.

Using the diagram of a toaster below I can show you the difference in a correctly wired and a reversed polarity outlet. The left toaster is wired correctly with the circuit being live only up to the switch. The right toaster with reverse polarity has all the wiring live up to the switch. With the toaster reversed if you were to put a knife or fork into it you would get shocked. This also works for a light bulb socket, the metal shell would become electrified and cause a shock if touched. If the wiring is correct it is harmless to touch a lamp shell or stick something in a toaster.


Reverse polarity needs to be repaired and is usually simple, most of the time just switching the wiring around. If you are comfortable working with electricity it is a easy DIY project, if not call a licensed electrician. If the whole home wiring is reversed then the services of a licensed electrician will be needed.

The only way to know if your wiring has reverse polarity is to test it, the easiest and simplest way to test is to purchase an inexpensive three light circuit tester at any hardware store.



Bad Upgraded Outlets

Inspecting older homes the most common defect I find is bad electrical outlet upgrades. Usually the outlets are upgraded by homeowners or other non-qualified person to do electrical. The following is the verbiage I use in my report.

“At some time the electrical outlets have been upgraded from the original 2-prong outlets to modern 3-prong outlets without upgrading the wiring to the outlets. This presents an unsafe condition. The purpose of the third prong is grounding.”

Installing a 3-prong outlet on a 2 wire system is wrong and not up to any codes, but is happens in many older homes. The purpose of the ground is to provide an alternate path for the electrons to flow back to the main electrical panel. If there should be a short, the electrons do not have an alternate path back to the electrical panel without the grounding wire.

When your home has bad upgrades as a homeowner you have four choices:

  • Upgrade the wiring to new grounded wire
  • Install GFCI (ground fault) protection
  • Install new 2-prong ungrounded outlets
  • Know the 3-prong outlets are not grounded and use as a 2 prong outlet with no ground. This condition needs to be disclosed to anyone using the outlet. The 3-prong outlet will give a false sense of security about the outlet being grounded.

If you use non-grounded outlets I recommend using a good quality surge protector for sensitive electronics. ( Desk top computers, flat screen tv’s, ect..)

C:Documents and SettingsGilbertMy Documentsinfo4b.pdf